Lithium-air cells, which use the porous carbon as their cathode material, can be considered the ‘holy grail’ of lithium batteries because they offer, in principle, a significantly superior theoretical gravimetric energy density approaching that of gasoline. It is this exceptional energy potentiality that has triggered the worldwide interest in such batteries.
A reversible, non-aqueous Li-air battery uses a lithium metal anode, a liquid organic electrolyte, and a carbon-supported metal-based catalyst air cathode. Li-air cells differ from conventional battery systems such as lead-acid, nickel-metal hydride, and lithium-ion systems, because oxygen is supplied as a fuel to the cell during discharge. During operation, the lithium metal anode releases an electron to the external circuit producing lithium ions in the electrolyte as the oxygen is reduced at the cathode surface to form lithium peroxide (Li2O2), lithium oxide (Li2O), or a combination thereof.
Among the major challenges for the development of Li-air batteries is finding solutions to the large charge overpotential and the irreversibility of Li—O2 reactions. Several types of catalysts, including metals, metal complexes, and metal oxides, have been examined for the electrochemical reactions in the Li-air cells, showing large differences in charge potential and capacity among different catalysts. However, in general, these charge potentials are still too high (>3.8V) for practical use of the Li-air battery.